Method of Transmitting Messages

ABSTRACT

To provide a method of transmitting messages, using a transmission protocol having time-slots, that allows the messages to be transmitted in a flexible way, it is proposed that each message has a message identifier assigned to it, that the messages are placed in order in a queue, and that the queue has a set of slot identifiers assigned to it.

The invention relates to a method of transmitting messages using atransmission protocol having time-slots.

In communications systems, which are usually formed by a plurality ofnodes arranged in the form of a network, messages are exchanged betweenthe individual nodes, to enable, amongst other things, information to bepassed on. Communication systems of this kind may for example be used tocontrol complicated technical arrangements having a plurality ofsensors, actuators and the like, preferably from a central control unit.Systems of this kind are used in particular in automotive engineering toenable, for example, signals from an acceleration sensor to be directedto a central control unit. If the acceleration sensor senses that alimiting value of acceleration is being exceeded, a signal to thiseffect is passed on to the control unit, which latter then, among otherthings, triggers an airbag. It is particularly necessary in this casefor safety-related messages or information to be passed on reliably.Access to a transmission medium or a channel, in the form of anelectrical signal lead for a node for example, is controlled by a mediaaccess control (MAC) protocol.

Particularly in automotive engineering, there are new types oftransmission protocols that are being used for message transmission,such as for example the protocol that is known to the person skilled inthe art as FlexRay. This protocol uses time-multiplex data transmissionframes, i.e. a cyclic repetition of the message transmissions, which aredivided into two different segments, namely a static one and a dynamicone. What is used in the static segment is medium access control basedon time-slots of a fixed length, whereas what is used in the dynamicsegment is a scheme based on priorities. The term time-slot is one knownfrom TDMA systems that is derived from the pattern over time in whichthe individual users, i.e. nodes, are given the exclusive right totransmit (media access) under the time-multiplexing process. In thepresent case, the term time-slot will also be applied in what follows tothe interval of time for which a node receives or claims the right totransmit e.g. in the dynamic segment of FlexRay systems or even of CANsystems. In CAN systems, the allocation of the right to transmit resultsfrom priorities and a no-loss arbitration, while in the dynamic segmentof FlexRay it is by a mini-slotting process that controls media accessby means of a virtual token. If one of the nodes has the right totransmit, it is able to dispatch a message of variable length, i.e. inthis case the term “time-slot” does not necessarily mean a constantinterval of time.

Each time-slot is identified by what is termed a slot identifier (SID)in the form of a number, the numbers beginning for example at SID=1 atthe beginning of a cycle. The fixed number of time-slots that a cyclehas is for example six, i.e. it has an SID range from 1 to 6.

For this purpose it is known for there to be provided, in addition, in abuffer store in which a message or data to be transmitted is stored, ameans of registering the time-slot that is assigned, to enable thestored messages to be transmitted in the time-slot laid down. In thisway, the message to be transmitted by this buffer store is alwayscoupled to a given time-slot.

This method is particularly advantageous for messages that constantlyrecur and that are emitted for example by one node on the network. Thesemessages may for example be a signal giving the current speed of themotor vehicle that is to be passed on to a speedometer. Because of thefixed time-slot, it is thus already known, in theory, in a componentthat receives the message, such as for example in the central controlunit or host of the communications system, from which node the messagehas been received or what kind of message is being received.

However, particularly in the dynamic segment of the FlexRay protocol,there may be hold-ups to messages because the messages which enter theFIFO buffer store for the transmission path are dealt with in successionand for this there is always only one particular time-slot available,which means that a message of lower priority that enters the bufferstore before a message of higher priority may block the latter untilsuch time as the time-slot for the message of lower priority opens. Whatthis means is that a message that is the first to arrive at the bufferstore is also the first to be passed on again.

DE 103 29 179 A1 describes a method, in particular one employing theFlexRay protocol, for managing a store between a communicationscontroller and a host processor, which is referred to as such in thepatent, belonging to a node on a network, each data message beingassigned an identifier and there being additional control data thatincludes a pointer to the data associated with the identifier.

GB 2 389 492 A describes a method of transmitting messages employing atime-slot assisted transmission protocol, in which each time-slot has anidentifier assigned to it. Each of the messages to be transmitted, i.e.a data transmission frame, has assigned to it a check value that ischecked in the receiver to ensure that the message has been correctlytransmitted.

US 2003/0099556 2 A1 discloses a method of transmitting messages on awireless network with the help of time-slots, in which certaintime-slots are assigned to a node on the network. If a node indicatesthat it requires more time for message transmission, a control means isable to allocate free time-slots to this node.

It is an object of the invention to provide a method of transmittingmessages of the kind specified in the opening paragraph, in whichmessages stored in a buffer store are not blocked unnecessarily due tothe fact of messages being assigned to free time-slots in an inflexiblemanner.

This object is achieved by virtue of the features specified in claim 1.

The key concept of the invention lies in the fact of the message to betransmitted, and a slot identifier that gives the position in the mediaaccess control, being decoupled from one another. This applies forexample to the static segment of the FlexRay protocol or totime-multiplex-based systems in general. The way in which this isachieved is that each message, i.e. the data to be transmitted or a datatransmission frame, has assigned to it a message identifier (MID) thatcan be distinguished from a slot identifier (SID). The messages are thenplaced in order in a transmit queue (TxQueue), being prepared in thisway for transmission by the central control unit or host. This placingin order in the queue is preferably effected with the help ofapplication software.

This queue is then assigned a set or group of slot identifiers to enablethe messages to be transmitted one after the other on the network. Theseslot identifiers are of a preset size that depends on the transmissionprotocol that is selected in the given case. The dynamic assignment ofthe next message in the queue to the next available time-slot in thegroup of slot identifiers assigned is preferably effected by circuitrythat is implemented.

The time-slots are then used in the transmission protocol for thetransmission of the messages, the messages from the queue beingtransmitted one after the other in the successive time-slots that arepreset by the group of slot identifiers. It is preferable for thispurpose for all the messages to be of the same length. Any desiredmessages are assigned to any respective desired time-slots in this case,which means that a receiver of the message, in particular, is no longerable to draw any conclusions as to the sender or the nature of themessage simply from the time-slot.

It goes without saying that, to allow the method to be performed, anetwork having nodes and a central host will be so designed in terms ofhardware and/or software that the messages to be transmitted can beprovided, either at the host or at each node, with a message identifier,can be placed in order in a queue and the queue can have assigned to ita set of slot identifiers, to make it possible for messages to betransmitted in the desired way from the host to the nodes or in theopposite direction, or between the nodes themselves. What are providedfor this purpose are, in particular, respective FIFO buffer stores,referred to below simply as buffer stores for short, for the messagesthat have been arranged into a queue.

The advantage of the invention lies in the fact that a given bandwidthof a transmission medium, i.e. a channel for the transmission ofmessages, is provided for a message transmission, rather than messagesbeing tied to a given time-slot in a fixed way, which means that a queuewill not be blocked by a message of lower priority that is stored in abuffer store until such time as the time-slot assigned to the messageopens.

Advantageous embodiments of the invention are characterized in thedependent claims.

The way of handling the queue that is specified in claim 2 ensures thata message that is the first to be read into a buffer store or that isstored there first will also be dispatched first. The result of thisfirst-in, first-out method is that although it is ensured that theintended sequence of transmission of the messages is preserved, theexact time at which the message is actually transmitted is not laid downin advance, because what is used for the next outgoing message is, ineach case, the next open time-slot that is available to the node.

In addition, each message may have a lapsing time assigned to it, asclaimed in claim 3. This lapsing time specifies the maximum lifespan ofthe message to be transmitted. If this lifespan has elapsed before themessage is transmitted, because for example a suitable time-slot has notyet been opened, the message is automatically deleted, to enable thecapacity of the network not to be overloaded by outdated messages. In amotor vehicle, a message of this kind may for example be a lesssignificant sensor signal such as an indication of outside temperature,the updating of which does not need to take place at fixed intervals oftime. It goes without saying that for this purpose the nodes and thehost have respective internal clocks that are synchronized with oneanother assigned to them, or the entire network is connected up to onecentral clock.

It is of course possible, as claimed in claim 4, for messages to beexchanged between a plurality of nodes, and/or a host, that togetherform a network. For this purpose, there are provided at the nodes,amongst other things, buffer stores for the queues of messages and meansfor transmitting and receiving the messages and for reading out themessage identifiers.

It is proposed in claim 5 that the message transmission takes place inaccordance with the FlexRay protocol, in which case use may be made ofboth the static and the dynamic part of the protocol. It is of advantagein this case that the message identifier is independent of the slotidentifier in a receiving node or host, because the messages providedwith a message identifier can be transmitted in any desired time-slots.Identification is performed simply by looking at the message identifier,which means that a node or host can detect from which other node or hostthe message was transmitted. The nodes may for example be numberedserially for this purpose.

It is possible in this case, as specified in claim 6, for the host forexample to transmit a larger number of messages with a number oftime-slots which is smaller than the number of messages. What this meansis that the messages in the queue waiting to be transmitted aretransmitted in succession in a plurality of cycles of, for example, theFlexRay protocol. Two messages per cycle for example may be transmittedin the dynamic segment of the FlexRay protocol, which means that twocycles are required for the transmission of four messages.

In the above, the invention has been described as essentially for thetransmission of messages from a plurality of components or sensorsand/or control means in a motor vehicle, as characterized in claim 7.However, it will be clear to the person skilled in the art that themethod may also be used in networks of any other kind between aplurality of nodes and, where required, a host, in which case electricalconductors, glass-fiber cables and also wireless radio links may be usedas channels for the transmission of messages.

It is also possible for the proposed method to be applied to othertransmission protocols for which it is advantageous for the messageidentifier to be decoupled from the slot identifier, particularly whenthe occurrence of the slot identifier is determined by priority-basedcontrol, as it is for example in the case of a controller area network(CAN) protocol.

The proposed method of decoupling the message identifier from the slotidentifier to assist queues on the transmission path of a node may alsobe combined, in the same node, with conventional methods of storingmessages and with a fixed way of assigning a dedicated message store toa time-slot. Depending on the requirements to be met by the application,the emission, under the deterministic control of time, of messages whoseassignment is fixed can be supported, as also can the simplifiedemission of messages that merely respects their sequence.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 shows the layout of a network, in diagrammatic form.

FIG. 2 shows a pattern in which message transmission takes place overtime.

The embodiment shown in FIG. 1 is a network comprising five nodes A, B,C, D, E and a channel 1 for the transmission of messages between thenodes A, B, C, D, E, such as is used for example in automotiveengineering for communication between a plurality of components of themotor vehicle. In principle, it is also possible for there to be twochannels or other channel topologies.

From the representation in FIG. 2 the pattern in which messages aretransmitted over time can be seen. Basically, the FlexRay protocol isdivided into a static segment SID1 to SID3 and a dynamic segment SID4 toSID6. Regardless of the number of time-slots m, a number of messages nis exchanged, the normal case being that n>m, e.g. it being selectedthat m=6 and n=15. For the sake of simplicity, it is assumed that allthe messages are of the same length and that the dynamic segment is ableto hold up to 2 messages per cycle.

Whereas there is a fixed correlation between MID1-3 and SID1-3 in thestatic segment, it is assumed that there is not a fixed correlation ofthis kind in the dynamic segment. The message identifier at the receiveris not dependent on the SID used for the MAC. Hence the MID fields thathave been standardized for the FlexRay protocol, or other encodings, maybe used for the transmission of messages.

The method is represented by way of example in the following table:

Maintained Node ID IDs (MID, SID) A MID 04 05 06 07 08 09 10 SID 05 06 BMID 11 12 13 14 15 SID 04 C MID 01 SID 01 D MID 02 SID 02 E MID 03 SID03

In this example, SID5 and SID6 are allocated only to node A and SID4only to node B. Node A emits messages MID4 to MID10 and node B emitsmessages MID11 to MID15. The other nodes share the static segment: nodeC is allocated SID1 and MID1, node D is allocated SID2 and MID2 and nodeE is allocated SID3 and MID3.

Where the buffer stores at nodes A and B have moved to the “full” stateas shown by way of example, FIG. 2 shows the resulting flow of messageson the channel in the individual successive communication cycles. Theassignment of the messages to the time-slots in the static segment (SID1-3) is fixed (MID 1-3) and is therefore not shown in detail in FIG. 2,as indicated by xxx.

LIST OF REFERENCE NUMERALS

-   A, B, C, D, E Nodes on a network-   l Channel for message transmission-   n Set of message identifiers (MID)-   m Set of slot identifiers (SID)

1. A method of transmitting messages using a transmission protocolhaving time-slots, characterized in that each message has a messageidentifier assigned to it, the messages are placed in order in a queue,and the queue has a set of slot identifiers assigned to it.
 2. A methodas claimed in claim 1, characterized in that the queue is handled by theFIFO method.
 3. A method as claimed in claim 1 characterized in that alapsing time is assigned to each message.
 4. A method as claimed inclaim 1, characterized in that messages are transmitted between aplurality of nodes, and/or a host, on a network.
 5. A method as claimedin claim 1, characterized in that the messages are transmitted inaccordance with the FlexRay protocol.
 6. A method as claimed in claim 1,characterized in that a larger number of messages are transmitted with asmaller number of time-slots.
 7. Use of the method as claimed in claim 1in automotive engineering.